Effect of carbon to nitrogen ratio and aeration rate on phosphorus and exchangeable cation contents and their leaching in the soil during olive pomace and turkey manure co-composting

The environmental issues related to olive oil by-products and turkey manure are continuously increasing. This ecological hazard could be mitigated by the composting process. This study aimed to investigate the effect of the initial carbon to nitrogen ratio (C/N) and aeration rate (turning frequency (TF)) on mineral contents (P, K, Ca, and Na) and their leaching in the soil surface. Olive pomace (OP) and turkey manure (TM) were co-composted to prepare six mixtures at three levels of initial C/N (20, 22, and 28) and two levels of TF (once and twice a week). The results revealed a substantial effect of TF, twice a week, resulting in a loss of 36.5%, 36%, and 27% for K, Ca, and Na contents, respectively, whereas TF, once a week, preserves the maximum of nutrients in heaps with good compost maturity. The initial C/N of 28 had significantly reduced P and K by 14% and 13%, respectively, and had increased Ca and Na contents by 85% and 30 %, respectively. The leaching of almost all the studied minerals has been demonstrated after composting. Exchangeable cations have been leached more under heaps with higher C/N ratios with TF of once per week, whereas P leaching has been recorded at the maximum level under the heap with an initial C/N of 20 and turned twice weekly. The resulting composts have generally displayed good mineral quality.